Current navigation systems include comprehensive databases in which, for instance, data are stored of a digitized map, data for inputting the destination, for route search and display and for destination guidance while indicating the current position of a vehicle. Furthermore, the databases may include additional data for driver-assistance systems.
The user of a database is interested in having the information included in the database in its most recent form, if possible. For instance, the value of a navigation database is reduced for the driver if the road network forming the basis of the database has changed or if a temporary traffic disturbance cannot be taken into account because the navigation database lacks pertinent information. Another example are computer applications that rely on current data.
A decentralized database is usually updated in such a way that the availability of new data is checked in a data center and a new version of the database is provided, if appropriate. When a new version of the database is available, then they are compiled and stored in an update database. The decentralized system, e.g., the navigation system in the vehicle, downloads suitable updated data from this updating database and integrates it into its decentralized database.
The updating of a database may be implemented in full or in part.
A complete update offers the advantage that the entire data stock is mutually adapted. However, it is disadvantageous that a complete update frequently entails unnecessary cost and time for the user. This is the case, for example, when data are transmitted that either have not changed or which are unnecessary for the user and the cost depends on the data volume.
To reduce the data quantities arising in connection with an update step, it is therefore not possible to update the entire database in a single step, but it is always updated only in part. To this end, the content of the database is subdivided into individual segments, e.g., according to geographical and/or thematic considerations or a combination thereof.
However, problems may arise in the updating implemented in incremental steps because the database may lose its consistency. That means that possible relationships with other segments are severed by the updating of a partial region. It is conceivable, for example, that in an update a street segment is deleted in one segment, to which another, not updated segments still refers by a street connection. Such inconsistencies must be avoided in an update in incremental steps.
A common procedure for avoiding inconsistencies in the update of a database is to subdivide the database into segments that do not overlap, each of which is then assigned a version identifier. Due to the dependencies between individual segments of the database, not every combination of versions will lead to a consistent state. Therefore, the data center and/or the decentralized system must ensure that each updating of the decentralized database results in a consistent combination of versions. The time and expense involved in this safeguarding increases with the number of dependencies among the individual segments. In an effort to reduce such dependencies, the particular segments of the database that must be checked in an update operation are formed in such a way that the number of relationships between the segments is minimized.
Another problem in an updating of the database in incremental steps is what is known as the avalanche effect. This means that, despite the user's wish to update merely one segment of the database, many versions of many segments of the database must be updated as well in order to obtain a consistent combination of versions. This in turn results in increased expense and an increased time requirement.
Avalanche effects may be contained by subdividing the database into smaller segments. However, smaller segments of the database whose versions must be checked result in a larger number of such segments. This also increases the required maintenance and the required memory space for the version control. In addition, the likelihood of what is known as structural changes is higher, which are to be understood as the addition or deletion of segments of the database whose versions are checked. Structural changes can be administered only with the aid of auxiliary structures.